BELT DRIVES _III Design of machine elements -II.pptx

Drives

1. Power Transmission

2. POWER TRANSMISSION Rotating elements which possess mechanical energy has to be utilized at required place by transmitting. From prime mover to machine From one shaft to another

3. Means of power transmission Gear Train Ex: Automobiles, engines etc. Rope Drive Ex: lifts, cranes etc. Chain Drive Ex: Bi-cycle, Motor cycle etc. Belt Drive Ex: Rice mills, Sewing machines etc.

4. 7 WHAT ARE BELT DRIVES? The amount of power transmitted depends upon the following factors : The velocity of the belt. The tension under which the belt is placed on the pulleys. The arc of contact between the belt and the smaller pulley. A belt drive is a method of transferring rotary motion between two shafts. A belt drive includes one pulley on each shaft and one or more continuous belts over the two pulleys. The motion of the driving pulley is, generally, transferred to the driven pulley via the friction between the belt and the pulley.

5. Belt drive selection criterion Speed of the driving and driven shafts, Power to be transmitted, Positive drive requirements, Space available, Speed reduction ratio, Centre distance between the shafts, Service conditions

6. Types of belt drives Light drives : small powers upto 10 m/s Ex: small machine tools, agricultural machines Medium drives : medium powers upto 22 m/s Ex: machine tool Heavy drives : large powers over 22 m/s Ex: compressors, generators

7. Types of Belts: 1. Flat belt : F actories and workshops- moderate amount of power - < 8 m apart 2. V-belt : F actories and workshops- moderate amount of power – very near 3. Circular belt or rope : F actories and workshops- great amount of power - > 8 m apart

8. Belt material Rubber Leather Cotton Balata – instead of rubber balata gum is used 11

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10. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 13

11. Belt Joints

12. Classification of Flat belt drives Open belt drive Closed belt drive www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 15

13. Open Belt Drive Both driver and driven pulley rotate in both direction. Driver pulley pulls the belt from one side and delivers to other side. Tension is more in lower side then upper side. 16

14. Cross Belt Drive Driven rotates in opposite direction to that of driver. At the point where the belt crosses it rubs against each other and there will be wear. To avoid this, speed of belt should be less than 15 m/s. 17

15. Quarter Turn Belt Drive Shafts arranged at right angles and rotates in one definite direction

16. Quarter turn belt drive

17. Belt Drive with Idler Pulleys Also known as Jockey pulley drive To maintain small angle of contact

18. Compound Drive Transmit through a number of pulleys

19. Stepped or cone pulley drive Changing the speed of driven when driving shaft is constant Fast and loose pulley drive To start or stop without interfering

20. Comparison between Open belt drive and Close belt drive Open Belt Drive Closed Belt Drive Both driver and the driven rotates in the same direction Driver and driven rotates in opposite direction When the shafts are horizontal, inclined it is effective to transmit the power Even if the shafts are vertical it is effective to transmit the power As there is no rubbing point, the life of the belt is more Due to the rubbing point, the life of the belt reduces. Require less length of the belt compared to crossed belt drive for same centre distance, pulley diameters. Require more length of belt compared to open belt drive for the same centre distance, pulley diameters. www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS 23

21. Velocity Ratio Ratio between the velocities of driver and driven

22. Length of Belt Drive Length of open belt L = Arc GJE + EF+ Arc FKH+ GH = 2(Arc JE+EF+ Arc FK) L

23. Length of Cross Belt Drive Length of cross belt L = Arc GJE + EF+ Arc FKH+ GH = 2(Arc JE+EF+ Arc FK) L

24. 27 KEY EQUATIONS OF BELT DRIVE SYSTEMS Length of the cross belt (m) = Length of the open belt (m) = Velocity of Belt (V) = Power (P) = T x V = Here ( d 1 =diameter in meters, N 1 =Speed in RPM, P 1 =power in Watts, T= Tension in the belt N ) Watts

25. Power transmitted by a Belt Work done per second = ( T 1 – T 2 ) V Power transmitted = ( T 1 -T 2 ) V Ratio of driving tensions for Flat Belt Drive T 1 / T 2 = e μθ Θ = Angle of lap or contact Centrifugal tension is to be taken into account when velocity is more than 10m/s. T C = m.v 2

26. 29 APPLICATIONS A OPEN BELT DRIVE IN A JIG-SAW MACHINE LATHE MACHINE WITH SPEED CONES AND TIMING BELT

27. Problems to be referred compulsorily For flat belt drives Example problems: 18.4, 18.7, 18.10, 18.11, For V belt drives: Examples are 20.1, 20.2, 20.3

28. Transmissions - summary Characteristic Friction wheel Spur gears Flat belts Trapezoidal belts Toothed belts Chains Max power [kW] 80 80e3 200 350 120 400 Max torque [kNm] 5 7000 3 5 1 40 Max linear speed [m/s] 20 20 100 30 60 10 Efficiency 0.95 0.97 0.97 0.97 0.96 0.95 Power function os speed y n y y y y Max ratio (1 stage) 6-18 6-10 6-8 6-10 6-10 6-10 Tensioning required y n y y n n Load on bearing high low high high low low Build precision average high low low low average Presence of sliding y n y y n n Noise low average low low low high Overload limiter y n y y n n Cost low high low average average average